This invention relates to Zener diodes and more specifically, to Zener diodes having a subsurface breakdown region connected to a temperature compensating device.
Zener diodes having subsurface breakdown regions are known in the art. Examples of such Zener diodes are discussed in U.S. Pat. Nos. 4,136,349 issued to Tsang, 4,079,402 issued to Dunkley, et al., 4,398,142 issued to Beasom and 4,319,257 also issued to Beasom, each of these patents being incorporated herein by reference. One reason for confining Zener breakdown to a subsurface region is to avoid drift in the Zener breakdown voltage with time. As is known in the art, a Zener diode exhibiting surface breakdown is subject to breakdown voltage drift because of the presence of a passivating or insulating silicon dioxide layer which typically lies adjacent the silicon surface where the breakdown takes place. This effect is discussed in greater detail in the above-mentioned patents. Accordingly, by causing Zener breakdown to occur at a subsurface region, breakdown voltage drift can be avoided.
It is also known in the art that the Zener breakdown voltage varies with temperature. Accordingly, it is known to couple a Zener diode to a temperature compensating device so that the voltage at the diode cathode relative to ground is relatively constant with respect to temperature. U.S. Pat. No. 4,398,142 discusses a circuit including an NPN transistor connected to a Zener diode to maintain a constant voltage at the Zener cathode. U.S. Pat. No. 4,319,257 illustrates another arrangement for coupling a Zener diode to an NPN transistor in order to compensate for Zener breakdown variation in response to temperature changes.